This invention relates to X-Y plotters, and in particular to an adaptive servo-controlled pen-lift system for raising and lowering a pen relative to a platen in such a plotter.
Generally, the pen-lift system in a high quality plotter becomes as important as the lateral positioning system if plots of drafting quality are to be obtained. For example, even if the lateral positioning system moves the pen accurately in a straight line, a drafting quality line will not result if there is uncontrolled pen bounce or if there is an incorrect pen force.
In the past, pen-lift systems have typically used a spring system to hold the pen to the recording medium and platen with a prescribed force, that force depending on the weight of the pen and pen carriage, the effective force constant of the spring system, and the pen-lift height. Attempts have been made to control pen bounce through spring damping systems of various kinds. In most implementations, the complications inherent in such mechanically intensive systems have led to two-state pen-height system, i.e., pen-up or pen-down.
It is important to note that such two state systems are manifestly inefficient in the often encountered plotting situation, e.g., lettering or drafting dashed lines, in which many pen lifts are required. Despite the fact that most pen strokes in these situations are in close proximity, the pen travels to its full height between successive separate strokes. Hence, a substantial amount of time is spent raising and lowering the pen rather than in plotting.
To date, no available plotting system has been developed which utilizes a feedback control system adaptive to platen height irregularities to control pen bounce and to increase the plotting speed by controlling pen-lift height.